The present invention relates to improved methods and apparatus for processing surfaces of glass substrates to provide low roughness and low, uniform waviness over the substrate surface. The invention has particular utility in surface preparation (i.e., polishing) of disk-shaped glass or glass-based substrates for use in the manufacture of magnetic data/information storage and retrieval media, e.g., hard disks.
Magnetic recording media are widely used in various applications, particularly in the computer industry. A portion of a conventional recording medium 1 utilized in disk form in computer-related applications is schematically depicted in FIG. 1 and comprises a non-magnetic substrate 10, typically of metal, e.g., an aluminum-magnesium (Alxe2x80x94Mg) alloy, having sequentially deposited thereon a plating layer 11, such as of amorphous nickel-phosphorus (NiP), a polycrystalline underlayer 12, typically of chromium (Cr) or a Cr-based alloy, a magnetic layer 13, e.g., of a cobalt (Co)-based alloy, a protective overcoat layer 14, typically containing carbon (C), e.g., diamond-like carbon (xe2x80x9cDLCxe2x80x9d), and a lubricant topcoat layer 15, typically of a perfluoropolyether compound applied by dipping, spraying, etc.
In operation of medium 1, the magnetic layer 13 can be locally magnetized by a write transducer or write head, to record and store data/information. The write transducer creates a highly concentrated magnetic field which alternates direction based on the bits of information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium layer 13, then the grains of the polycrystalline medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The pattern of magnetization of the recording medium can subsequently produce an electrical response in a read transducer, allowing the stored medium to be read.
Thin film magnetic recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (xe2x80x9cCSSxe2x80x9d) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, allowing data to be recorded on and retrieved from the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in air, sliding against the surface of the disk, and stopping.
It is considered desirable during reading and recording operations, and for obtainment of high areal recording densities, to maintain the transducer head as close to the associated recording surface as is possible, i.e., to minimize the xe2x80x9cflying heightxe2x80x9d of the head. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk surface to be positioned in close proximity, with an attendant increase in predictability and consistent behavior of the air bearing supporting the head during motion.
Meanwhile, the continuing trend toward manufacture of very high areal density magnetic recording media at reduced cost provides impetus for the development of lower cost materials, e.g., polymers, glasses, ceramics, and glass-ceramics composites as replacements for the conventional Al alloy-based substrates for magnetic disk media. However, poor mechanical and tribological performance, track mis-registration (xe2x80x9cTMRxe2x80x9d), and poor flyability have been particularly problematic in the case of polymer-based substrates fabricated as to essentially copy or mimic conventional hard disk design features and criteria. On the other hand, glass, ceramic, or glass-ceramic materials are attractive candidates for use as substrates for very high areal density disk recording media because of the requirements for high performance of the anisotropic thin film media and high modulus of the substrate. However, the extreme difficulties encountered with grinding and lapping of glass, ceramic, and glass-ceramic composite materials have limited their use to only higher cost applications, such as mobile disk drives for xe2x80x9cnotebookxe2x80x9d-type computers.
As employed herein, the term xe2x80x9cglassxe2x80x9d is taken to include, in the broadest sense, non-crystalline silicates, aluminosilicates, borosilicates, boroaluminosilicates, as well as polycrystalline silicates, aluminosilicates, and oxide materials; the term xe2x80x9cceramicxe2x80x9d is taken to include materials consisting of crystalline particles bonded together either with a glass (i.e., vitreous) matrix or via fusion of the particles at their grain boundaries, as by sintering, as well as refractory nitrides, carbides, and borides when prepared in the form of bodies, as by sintering with or without a glass matrix or a silicon- or boron-containing matrix material, e.g., silicon nitride (Si3N4), silicon carbide (SiC), and boron carbide (B4C); and the term xe2x80x9cglass-ceramicsxe2x80x9d is taken to include those materials which are melted and fabricated as true glasses, and then converted to a partly crystalline state, such materials being mechanically stronger, tougher, and harder than the parent glass, as well as non-porous and finer-grained than conventional polycrystalline materials.
As indicated supra, glass and glass-ceramic materials are attractive candidates for use as substrates for magnetic data/information storage and retrieval media, e.g., hard disks. However, the extreme difficulties encountered with the surface preparation of such materials, e.g., grinding, lapping, polishing, etc., have heretofore limited their use to only higher cost applications, such as mobile disk drives for xe2x80x9cnotebookxe2x80x9d-type computers. Further, existing systems for polishing glass or glass-ceramic materials for use as substrates for magnetic recording media do not provide polishing platforms with adequate capability for current disk drive technology and requirements, particularly with respect to substrate micro-roughness, waviness, and uniformity. In other instances, existing systems for polishing glass or glass-ceramic materials for use as media substrates provided polished surfaces free of imperfections but did not planarize or polish the surfaces to a degree compatible with the increased areal recording densities of current mechanical disk drive systems.
In view of the foregoing, there exists a clear need for improved means and methodology for providing high modulus glass or glass-based substrates for magnetic data/information storage and retrieval media, e.g., disk-shaped substrates, with at least one surface thereof having requisite topography, i.e., low waviness over the entire surface together with lower average roughness, for enabling operation of the media with read/write transducers/heads operating at very low flying heights.
The present invention addresses and solves problems and difficulties attendant upon the surface preparation of very hard, high modulus materials, e.g., glasses, ceramics, and glass-ceramics, for use as substrate materials in the manufacture of very high areal density magnetic recording media, while maintaining full capability with substantially all aspects of conventional automated manufacturing technology for the fabrication of thin-film magnetic media. Further, the methodology and means afforded by the present invention enjoy diverse utility in the manufacture of various other devices and media requiring formation of low waviness, low average surface roughness surfaces on high hardness materials.
An advantage of the present invention is an improved method of polishing at least one surface of a glass, ceramic, or glass-ceramic substrate.
Another advantage of the present invention is an improved method of polishing at least one surface of a glass, ceramic, or glass-ceramic substrate to minimize the waviness, the variation in waviness, and the average surface roughness of said at least one surface, whereby the substrate is usable as a substrate for a magnetic or magneto-optical (MO) data/information storage retrieval medium.
Yet another advantage of the present invention is an improved system for polishing at least one surface of a glass, ceramic, or glass-ceramic substrate.
A further advantage of the present invention is an improved system for polishing at least one surface of a glass, ceramic, or glass-ceramic substrate to minimize the waviness, the variation in waviness, and the average surface roughness of said at least one surface, whereby the substrate is usable as a substrate for a magnetic or magneto-optical (MO) data/information storage retrieval medium.
A still further advantage of the present invention is an improved polishing pad for polishing a surface of a glass, ceramic, or glass-ceramic substrate.
A yet further advantage of the present invention is an improved polishing pad for polishing at least one surface of a glass, ceramic, or glass-ceramic substrate to minimize the waviness, the variation in waviness, and the average surface roughness of said at least one surface, whereby the substrate is usable as a substrate for a magnetic or magneto-optical (MO) data/information storage retrieval medium.
Another advantage of the present invention is an improved method of manufacturing a polishing pad for use in polishing a surface of a glass, ceramic, or glass-ceramic substrate.
A yet further advantage of the present invention is an improved method of manufacturing a polishing pad for polishing at least one surface of a glass, ceramic, or glass-ceramic substrate to minimize the waviness, the variation in waviness, and the average surface roughness of said at least one surface, whereby the substrate is usable as a substrate for a magnetic or magneto-optical (MO) data/information storage retrieval medium.
Additional advantages and other aspects and features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to an aspect of the present invention, the foregoing and other advantages are obtained in part by a method of polishing at least one surface of a glass, ceramic, or glass-ceramic substrate to minimize the waviness, the variation in waviness, and the average surface roughness of the at least one surface, whereby the substrate is usable as a substrate for a magnetic or magneto-optical (MO) data/information storage retrieval medium, the method comprising sequential steps of:
(a) performing a primary polishing of the at least one surface of the substrate in a first polishing apparatus, utilizing a first polishing slurry containing particles of a first abrasive;
(b) transferring the substrate to a second polishing apparatus; and
(c) performing a final polishing of the at least one surface of the substrate in the second polishing apparatus, utilizing a second polishing slurry containing particles of a second abrasive, the particles of the second abrasive being smaller than the particles of the first abrasive.
According to embodiments of the present invention, steps (a) and (c) comprise utilizing first and second polishing apparatuses comprising respective first and second polishing pads each having a polishing surface treated to harden it and to minimize absorption of the particles of the first and second abrasives, i.e., steps (a) and (c) comprise utilizing first and second polishing apparatuses comprising respective first and second porous polishing pads including a ceramic or an amorphous glass material deposited on the polishing surface to harden it and to reduce the void area thereof without creating a hydrophobic condition.
Preferred embodiments of the present invention include those wherein steps (a) and (c) comprise utilizing first and second polishing apparatuses comprising respective first and second high density, formable polishing pads made of a polyurethane or woven material and the amorphous glass material deposited on the polishing surfaces is derived from at least one metal silicate.
According to preferred embodiments of the present invention useful in polishing substrate surfaces for use in manufacture of thin film magnetic and/or magneto-optical recording media, step (a) comprises removing up to about 50 xcexcm of glass, ceramic, or glass-ceramic material from the surface of the substrate to form a planar and uniform surface having an average roughness Ra of about 4 xc3x85 and a waviness of about 4 xc3x85; and step (c) comprises removing less than about 3 xcexcm of glass, ceramic, or glass-ceramic material from the surface of the substrate to form a planar and uniform surface having an average roughness Ra of about 1 xc3x85 and a waviness of about 2 xc3x85 over the entire surface; wherein step (a) comprises utilizing a CeO2-based abrasive slurry as the first polishing slurry; and step (c) comprises utilizing a colloidal SiO2-based abrasive slurry as the second polishing slurry.
In accordance with these preferred embodiments, step (a) comprises utilizing a CeO2-based first polishing slurry comprising CeO2 particles having sizes  less than 0.2 xcexcm; and step (c) comprises utilizing a colloidal SiO2-based second polishing slurry comprising SiO2 particles having sizes  less than 25 nm; wherein step (a) comprises utilizing a CeO2-based first polishing slurry wherein the size distribution of the CeO2 particles is  less than +/xe2x88x923 D50, D50 being the mean particle size at the centerline of the CeO2 particle size distribution, and wherein step (a) comprises utilizing a CeO2-based first polishing slurry comprising about 3.0 to about 5.0% by volume CeO2 solids; and step (c) comprises utilizing a colloidal SiO2-based second polishing slurry comprising about 9.0 to about 11.5% by volume colloidal SiO2 solids.
According to embodiments of the present invention, step (a) comprises utilizing a first polishing apparatus comprising means for recirculating the first polishing slurry, the recirculating means including filter means for removing particles of sizes equal to or greater than a pre-selected size from the first polishing slurry; step (c) comprises utilizing a second polishing apparatus comprising means for recirculating the second polishing slurry, the recirculating means including filter means for removing particles of sizes equal to or greater than a pre-selected size from the second polishing slurry; and step (b) comprises transferring the substrate in a wet state from the first polishing apparatus to the second polishing apparatus.
Another aspect of the present invention is a system for polishing at least one surface of a glass, ceramic, or glass-ceramic substrate to minimize the waviness, the variation in waviness, and the average surface roughness of the at least one surface, whereby the substrate is usable as a substrate for a magnetic or magneto-optical (MO) data/information storage retrieval medium, the system comprising:
(a) a first polishing apparatus for performing a primary polishing of the at least one surface of the substrate, comprising a first polishing slurry containing particles of a first abrasive;
(b) means for transferring the substrate from said first polishing apparatus to a second polishing apparatus; and
(c) a second polishing apparatus for performing a final polishing of the at least one surface of said substrate, comprising a second polishing slurry containing particles of a second abrasive, the particles of the second abrasive being smaller than the particles of the first abrasive.
According to embodiments of the present invention, each of the first polishing apparatus (a) and the second polishing apparatus (c) is a planetary polishing apparatus comprising a porous polishing pad having a polishing surface treated to harden it and to minimize absorption of the particles of the first and second abrasives.
Preferred embodiments of the present invention include those wherein each of the first polishing apparatus (a) and the second polishing apparatus (c) comprises a porous polishing pad including a ceramic or amorphous glass material deposited on the polishing surface to harden it and to reduce the void area thereof without creating a hydrophobic condition; wherein each of the first polishing apparatus (a) and the second polishing apparatus (c) comprises a high density, formable polishing pad made of a polyurethane or woven material and the amorphous glass material deposited on the polishing surfaces is derived from a metal silicate.
According to preferred embodiments of the present invention, each of the first polishing apparatus (a) and the second polishing apparatus (c) comprises means for recirculating the respective polishing slurry, each recirculating means including filter means for removing particles of sizes equal to or greater than a pre-selected size from the respective polishing slurry; and the means for transferring said substrate from the first polishing apparatus to the second polishing apparatus comprises means for transferring the substrate in a wet state.
Yet another aspect of the present invention is a polishing pad for use in polishing a surface of a glass, ceramic, or glass-ceramic substrate to minimize the waviness, the variation in waviness, and the average surface roughness of the at least one surface, whereby the substrate is usable as a substrate for a magnetic or magneto-optical (MO) data/information storage retrieval medium, comprising:
a porous polishing pad having a polishing surface treated to harden it and to minimize absorption of abrasive particles of less than a pre-selected size.
According to preferred embodiments of the present invention, the porous polishing pad comprises a ceramic or amorphous glass material deposited on the polishing surface to harden it and to reduce the void area thereof without creating a hydrophobic condition; wherein the polishing pad comprises a high density, formable polyurethane or woven material and said amorphous glass material deposited on said polishing surface is derived from at least one metal silicate.
Still another aspect of the present invention is a method of manufacturing a polishing pad for use in polishing a surface of a glass, ceramic, or glass-ceramic substrate to minimize the waviness, the variation in waviness, and the average surface roughness of the at least one surface, whereby the substrate is usable as a substrate for a magnetic or magneto-optical (MO) data/information storage retrieval medium, comprising sequential steps of:
(a) providing a pad of a high density, formable, porous material;
(b) placing the pad on a platen, the pad having an exposed upper surface;
(c) shaping/dressing the pad to eliminate imperfections on the exposed upper surface of the pad arising from the underlying platen; and
(d) forming a layer of a ceramic or amorphous glass material on the pad which coats and fills pores in the exposed upper surface.
According to embodiments of the present invention, step (d) comprises sequential sub-steps of:
(d1) saturating the pad with a liquid containing at least one amorphous glass material applied to the exposed upper surface of the pad;
(d2) drying the liquid to form a layer of the amorphous glass material at least partially filling the pores and coating the exposed upper surface; and
(d3) curing the layer of amorphous glass material at an elevated temperature.
Preferred embodiments of the present invention include those wherein step (a) comprises providing a pad composed of a porous polyurethane or a woven material; step (d1) comprises applying an aqueous liquid containing at least one metal silicate to the exposed upper surface of the pad; and step (d3) comprises planarizing/polishing the exposed upper surface of the pad at an applied pressure, rpm, and interval selected to compress the pad and generate sufficient frictional heat to effect curing of the layer of amorphous glass material at an elevated temperature; wherein step (d1) comprises spraying an aqueous liquid containing hydrated aluminum silicate and lithium silicate on the exposed upper surface of said pad; and step (d3) comprises planarizing/polishing the exposed upper surface of the pad utilizing a ceramic plate and a CeO2-based abrasive polishing slurry containing a caustic reducing agent.
A further aspect of the present invention is a polishing pad made according to the above process.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shoen and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.